Recent advances in synthesis and controlled assembly of monodisperse colloidal nanocrystals into superlattice structures have enabled their applications in optics (1), electronics (2), magnetic storage (3), solar energy, etc. Single- and multi-component superlattices composed of spherical nanocrystals have been extensively studied in a variety of aspects such as structural diversity (4, 5), and electronic (6) and magnetic (7) interactions between the constituents. On the other hand, significant efforts have been put into developing new synthetic approaches for non-spherical nanocrystals that often exhibit physical properties unobtainable by simply tuning the size of the particles (8-11). However, the organization and application of these anisotropic building blocks have been limited mainly due to the lack of sufficient control over size uniformity, shape selectivity, surface functionality and the scarcity of convenient and reliable assembly methodology.
One particularly interesting class of materials which have wide potential as monodisperse colloidal nanocrystals are inorganic luminescent or electromagnetically active materials, crystalline materials that absorb energy acting upon them and subsequently emit the absorbed energy. Light emission is known as luminescence. A luminescent material which continues to emit light for greater than 10−8 seconds after the removal of the absorbed light is said to be phosphorescent. Phosphorescent substances are also known as phosphors. The half-life of the afterglow, or phosphorescence, of a phosphor will vary with the particular substance and typically ranges from about 10−6 seconds to days. Phosphors may generally be categorized as stokes (down-converting) phosphors or anti-stokes (up-converting) phosphors. Phosphors which absorb energy in the form of a photon and emit a lower frequency (lower energy, longer wavelength) band photon are down-converting phosphors. In contrast, phosphors which absorb energy in the form of two or more photons in a low frequency and emit in a higher frequency (higher energy, shorter wavelength) band are up-converting phosphors. Up-converting phosphors, for example, are irradiated by near infra-red light, a lower energy, longer wavelength light, and emit visible light which is of higher energy and a shorter wavelength. Phosphors may also be categorized according to the nature of the energy which excites the phosphor. For example, phosphors which are excited by low energy photons are called photoluminescent and phosphors which are excited by cathode rays are called cathodluminescent.
Lanthanide-doped nanophosphors have become an emerging class of optical materials during the past few years (12). These nanophosphors often possess “peculiar” optical properties (e.g., quantum cutting (13) and photon upconversion (14)), allowing the management of photons that could benefit a variety of areas including biomedical imaging (15, 16) and therapy (17), photovoltaics (13, 18), solid state lightning (19), and display technologies (20). Colloidal upconversion nanophosphors (UCNPs) are capable of converting long-wavelength near-infrared excitation into short-wavelength visible emission through the long-lived, metastable excited states of the lanthanide dopants (21). In contrast to the Stokes-shifted emissions from semiconductor nanocrystals or organic fluorophores and the multiphoton process employing fluorescent dyes, UCNPs offer several advantages including narrow, tunable emission bands (22), non-blinking emission and remarkable photostablity (15, 23), good brightness under low power continuous wave laser excitation, low autofluorescence background and deep penetration depths in biological systems (15, 16), etc. It has been widely accepted that hexagonal phase NaYF4 (β-NaYF4) is one of the best host materials for upconversion due to its low phonon energies (24), being more efficient than the cubic, α-NaYF4 phase (25). Several chemical approaches including coprecipitation (26) and hydrothermal synthesis (27) have been employed to synthesize β-NaYF4-based UCNPs. However, these methods are usually limited by drawbacks such as the necessity of post-synthesis treatment to improve crystallinity of the products, long reaction time (ranging from a few hours up to several days) and the use of specialized reactors (e.g., autoclaves). The synthesis of lanthanide fluoride nanocrystals via the thermal decomposition of metal trifluoroacetate precursors has been described (28, 29). Preparations of β-NaYF4-based UCNPs through decomposition of mixed trifluoroacetates (30) or through a two-step ripening process using the premade α-NaYF4 nanocrystals as precursors (31) were subsequently reported. Other reported methods to prepare monodisperse inorganic phosphor particles include sol-gel methods (U.S. Pat. No. 5,637,258); fluidized bed methods (U.S. Pat. No. 6,039,894); and solution-precipitation of precursors followed by heating (U.S. Pat. No. 6,132,642). Despite these recent progresses, the feasibility of the synthetic approach and the quality of the as-synthesized UCNPs or other inorganic particles using existing recipes are still far from satisfactory. There remains a need for a straightforward synthesis of not only UCNPs but for luminescent or electromagnetically active inorganic particles in general.
UCNPs, as one example, and other inorganic phosphor particles as well, are employed in a wide variety of applications, for example, in labeling, 3-D volumetric displays and in diagnostic assays. See, e.g., U.S. Pat. Nos. 4,870,485; 5,943,160; 5,043,265; 5,698,397; and 7,858,396; and U.S. Published Application 2007/0247595. Upconverting and downconverting phosphors are also used in smaller niche areas such as cosmetic/cosmeceutical, printed art, vehicle automation and guidance, process control, among others.
Inorganic phosphors are used, for example, to label security documents or currency. Phosphor particles may be incorporated into a document, currency or other security articles and detected to determine its authenticity. Phosphors used in this way are often called “taggants.” Combinations of different phosphor particles with different excitation and/or emission wavelengths, are used to provide a unique security signature to the security document or currency, etc. See U.S. Pat. Nos. 7,030,371; 7,790,056; 7,927,511; and 7,999,237. Similarly, phosphor particles may be incorporated into bulk materials, such as raw ingredients, and provide a label or signature that identifies the source and/or integrity of the bulk material. See, e.g., U.S. Pat. Nos. 7,323,696 and 6,536,672.
Diagnostic assays employ phosphor particles as detection labels to show the presence or absence of a particular analyte of interest. The phosphor particles may be surface treated to enable them to bind to the biological components of the assay and/or the analyte of interest. Using phosphor particles with different excitation and/or emission wavelengths allow for multiplexing—the detection of more than one analyte in a single assay using different labels to identify particular analytes. See, e.g., U.S. Pat. Nos. 5,043,265; 5,698,397; and 7,858,396. Phosphor particles find numerous uses in vitro diagnostics, including point-of-care diagnostics, flow cytometry, high throughput screening for drug discovery, genetic analyses, or early detection of disease. Generally, the types of analyses in this area encompass; state of health, congenital diseases, progress or course of treatment, and but not limited to determination of compatibility in blood or organ donations and transplants and labeling of cellular markers for use in pathological analysis of specimens. There are applications across many fields of medicine for medical imaging in the diagnosis and tracking of disease and in therapeutics such as Photodynamic Therapy, a light-based cancer therapy used for various malignancies. For example, phosphor particles are also used in biomedical imaging and therapy where the phosphors attach to tissue or cells for use in both in vitro and in vivo testing as well as in treatment of disease such as cancer See, e.g., Xiong L, Yang T, Yang Y, Xu C, Yi F (2010) Biomaterials, 7078-7085, and Ungun B, Prud'homme R, Budijono S, et al. (2008) Optics Express 17(1), 80-86.
The differentiation among phosphors or combination of different phosphors as labels is dependent on the use of a combination of different phosphor based on phosphor composition. As described in U.S. Pat. No. 5,698,397, phosphors may use different combination of absorbers and emitters to create unique phosphors. For example, a plurality of phosphors may absorb the same wavelength of light but, by having different emitters, emit at different wavelengths. Alternatively, a plurality of phosphors may emit the same wavelength but absorb different wavelengths. Of course, the phosphors in a plurality of phosphors may each have its own unique absorption wavelength and unique emission wavelength. Thus, based on the particular phosphor composition, different absorption and/or emission profiles are possible. This compositional differentiation is needed when the phosphor particles are spherical or do not have a uniform shape distribution. This, in turns, places demands and constraints on the equipment used when combinations of phosphors are used to create a unique signature or to differentiate among analytes.